Controlled Synthesis and Characterization of Co-Au Core-shell Nanoparticles
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Controlled Synthesis and Characterization of Co-Au Core-shell Nanoparticles Yuping Bao, Hector Calderon1 and Kannan M. Krishnan* Materials Science and Engineering, University of Washington, Seattle, WA 98195-2120, U.S.A. 1 Depto. Ciencia de Materiales, ESFM-IPN, Mexico, DF 07338, Mexico ABSTRACT Co-Au Core-shell nanoparticles are synthesized by slowly reducing an organo-gold compound on pre-made cobalt seeds with a weak reducer at mild condition. For the first time, these coreshell nanoparticles are generated in non-polar solvent in a controlled manner. The formation theory of core-shell structure, especially the seed size effect, is addressed as well. These coreshell structures are confirmed with a wide range of transmission electron microscopy (TEM) methods, which includes routine TEM images, high resolution TEM, and z-contrast imaging. INTRODUCTION Magnetic nanoparticles (NPs) have potential applications in many biological and medical applications such as magnetic separation, hyperthermia treatment, magnetic resonance contrast enhancement and drug delivery. Even though biocompatible iron oxides NPs are currently the material of choice, there is significant interest in developing alternative high moment cobalt (and related alloys) for specific biomedical applications [1], which involve significant generation of forces, such as bio-separation and targeted delivery. However, the biological applications of cobalt NPs are limited by their poor biocompatibility and resistance to oxidation. It would be highly desirable to coat them with an inert shell for biocompatibility and stability. Further, if the shell could provide additional functionality, such as sensitivity to optical probes and other biomolecules, it would be highly desirable for a number of applications. Gold coating of the magnetic nanocrystals would be a natural choice to accomplish both of these goals. Core-shell nanoparticles have received considerable attention recently because their physical and chemical properties can be tuned by controlling their chemical composition and the relative sizes of the core and shell. Besides the dual functionality of core-shell structure, their specific performance can be also improved. For example, the Ag-Au core-shell structure, exhibits a strong Ag surface plasmon resonance band combined with ease of Au surface functionalization [2], CdSe-ZnS core-shell semiconductor materials show improved luminescence quantum yields [3], and bimagnetic FePt -Fe3O4 core-shell structures display strong exchange spring behavior and an enhanced energy product [4]. These physical and chemical properties of the core-shell structure strongly depend on the structure of the core, the shell and the interface. Gold coated magnetic nanoparticles have been reported by different groups [5-9]. However, in addition to reproducibility, the growth processes do not lend themselves to the production of sufficient quantities of uniformly coated core-shell particles, because the synthesis environment is rich in oxygen and the presence of water acc
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